1. Field of the Invention
The present invention relates to plastic injection molding systems and methods for producing a high quality surface finish, with plastic and energy reductions.
2. Background Art
When injecting molding plastic parts in a closed mold, it is typically necessary to provide a liquid cooling system for the mold to cool the mold and the hot plastic injected into the cavity so that the part cools within a reasonable time period. If the part to be made has very thin sections, the plastic may be cooled too rapidly in the mold resulting in the formation of flow lines or knit lines in the part surface. If the thin part is provided with any locally thick sections such as mounting bosses, a dissimilar rate of cooling can result in sink marks on the appearance side of the part. In an effort to minimize flow marks in thin part sections, some manufacturers have actually heated the cavity portion of the mold by circulating hot liquid through the coolant passages so that the plastic is injected into the hot mold to completely fill the cavity without causing any flow marks or visible net lines. Once the cavity is filled, then the hot fluid is substituted for a coolant to chill the mold and solidify the part. The alternate heating cooling cycles are fairly expensive increasing cycle time and cause a great deal of thermal stress on the mold structure and frequently only heat cycles the cavity portion of the mold.
An alternative way of heating the mold prior to injection is to open the mold and expose the mold cavity surface to a induction heat source. Once the mold cavity surface is sufficiently hot, the heat source is removed and the mold closed and injection is commenced. The use of an induction heater, again is expensive, creates significant space issues in the tightly confined mold area and significantly adds to cycle time and only applied to a relatively flat surface with shallow depth. Induction heating is frequently only applied to the cavity portion.
There have been some efforts to utilize high pressure steam to preheat a mold cavity. While steam is relatively inexpensive and can rapidly heat the mold cavity, problems have resulted from the condensate being trapped in the mold effecting part quality. This method did not remedy the sink problem over ribs and bosses. Accordingly, steam heating has not been readily adopted by the industry.